The invention relates to a device for the protection of an electrode during the resistance welding of workpieces, particularly metal sheets, including a strip placed over the electrode, preferably in a manner displaceable relative to the same, wherein a carrier material of the strip is provided with an electrically conductive coating.
At present, metal sheets or other workpieces made of aluminum alloys and magnesium as well as galvanized or coated steel sheets—partially in high-strength quality—are used to an increasing extent, for instance, in body-making. Apart from advantages like weight saving and good corrosion resistance, those materials or their coatings cause problems during joining by resistance spot-welding. Above all, the tool life quantity of the spot-welding electrodes employed, which is strongly reduced relative to that of blank steel sheets, has negative effects. The high wear of the electrodes involves high costs, requiring frequent reworking of the electrode contact surfaces and frequent electrode exchanges resulting therefrom, as well as a reduced welding quality, particularly prior to an electrode reworking process or an electrode exchange.
To protect spot-welding electrodes from contamination or electrode pick-up by the material to be welded, it is known to insert a metal film in strip-form between the electrode and the workpiece. In order to ensure safe strip feeding, it is necessary to prevent the strip from adhering to the electrode contact surface.
In this respect, a device for the protection of electrodes during spot-welding is known from EP 0 830 915 B1, in which a strip is drawn by the aid of an unwinding mechanism over the electrode to be protected. The strip consists essentially of a copper-nickel alloy, or pure nickel, and has a thickness of 0.02 to 0.05 mm. In order to ensure an extended tool life quantity, the electrode and, in particular, the electrode cap is coated with silver, or a silver metal-oxide, or provided with an appropriate insert.
Furthermore, a strip for the protection of an electrode in a resistance welding process is known from U.S. Pat. No. 5,552,573 A, which strip is comprised of a base material coated on either side. The layers may be comprised of the same or of different materials. The base material has a thickness of 0.02 to 1 mm and is comprised of iron, steel, copper or a copper alloy. The applied layers have thicknesses ranging from 1 to 100 μm and may be comprised of nickel, titanium, niobium, molybdenum, tungsten, chromium, cobalt or alloys thereof. Such strips have the drawback of involving extremely high production demands due to coating or alloying and, hence, extremely high production costs. Another very important disadvantage resides in that a plurality of combinations of the most different materials may be envisaged so that a demanding and expensive storage of such combination strips for the most different applications will be required, since users will not be able to compose the different combinations on their own.
Applications of strips for the protection of electrodes in spot-welding methods are further known from DE 197 54 546 C1, JP 10-029071 A, JP 08-118037 A, JP 04-322886 A or JP 05-192774 A. There, the strips, which are supplied and discharged via winding mechanisms, are positioned above the electrodes in the spot-welding tools. The strip will, thus, come to lie on the workpiece during a spot-welding process, and the electrode will, thus, be protected from contacting the workpiece.
In the methods according to DE 197 54 546 C1 and JP 04-322886 A, it is disadvantageous that the protective strip is drawn over the electrode during conveyance, thus causing a high wear of the electrode on account of the friction occurring between the electrode and the strip.
The methods according to JP 10-029071 A and JP 08-118037 A involve the disadvantage of requiring a highly complex structure for the conveyance of the strip and, in particular, its supply and discharge, so that spot-welding tools of this kind are hardly usable in practice. By such apparatus it is, in fact, only feasible to weld together simple metal sheets. Their application in spot-welding tongs for robots, particularly in the automotive industry, is not possible because this would require small and compact spot-welding tools. JP 07-051865 A describes a protective strip for electrodes in the resistance welding of workpieces, which strip is comprised of a carrier material having an electrically conductive coating arranged on either side. The carrier material consists of copper, while the electrically conductive coating is made of graphite particles. The coating of the copper foil with the graphite particles serves to render the alloying reaction between the workpiece and the electrodes, and the workpiece and the electrode protection layer, more difficult. The manufacture of such a strip is, however, relatively demanding.